What is D-STAR: ham radio digital voice

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D-STAR is a form of digital communications used for amateur radio. D-STAR stands for Digital Smart Technology for Amateur Radio and it is an open Amateur Radio standard for digital communications.

Although D-STAR is able to provide both data and voice communications, D-STAR is used primarily for voice, but the addition of data can provide some useful features.

Originally developed in the late 1990s by the Japan Amateur Radio League, D-STAR has been adopted by a number of manufacturers and there are not only simplex transmissions, but repeaters are also equipped for D-STAR communications.

D-STAR basics

D-STAR uses a data transmission based around the use of a modulation format called minimum shift keying, MSK. This provides a robust and spectrum efficient form of data transmission.

D-STAR enables users to connect via both digital voice data communications. The data may be either what is termed high or slow speed. Slow speed digital voice and data are transported at 4800 bps, of which 3600 bps is used for voice transmission, the remaining 1200 bps is used for synchronisation and general use. Of the available 1200 bps, around 900 bps is available for the transmission of data.

Typically the data capability is used for sending identifiers and similar useful information - for example pressing the PTT will normally trigger the sending of the callsign via the data capability. Other information can be enabled as required and could include the GPS location, etc.

The D-STAR voice and low-speed data signal offers a significant improvement in spectrum efficiency. It utilises a 6 kHz channel bandwidth instead of the much wider requirements needed for narrowband FM used on the VHF and UHF bands.

The digital nature of the voice means that high quality audio is provided - comparable with that of a fully quieting NBFM signal, and in addition to this there is no noise or squelch tail after the transmission ceases. As with any digital signal, it can be copied at lower strengths than an analogue signal of similar strength, but when it goes between a certain threshold, all audio is suddenly lost.

The voice encoding is provided by an AMBE codec which encodes the voice at 3600bps. This codec very robust, but it is proprietary and the cost of its use does add significantly to the radios using it. A further disadvantage is that whilst the majority of the D-STAR standard is open, this area is not as the codec is commercially owned and the source code is not available.

D-STAR Internet linking

D-STAR provides a host of features that enable communications that can be relayed over the Internet and using digital technology there is a far greater degree of flexibility and capability. It is possible to enter the callsign of a station, and then even if they are out of direct communications range, they can still be contacted.

There are many repeaters and these can be linked by users to form a wider conference area, or alternatively system administrators can link repeater gateways together to link all voice & data from multiple repeaters together.

It is also possible for repeaters to operate in a cross band mode, receiving on one band and transmitting on another.

Using digital communications technology, D-STAR is able to provide a far more comprehensive form of communications rather than the point to point communications traditionally established in amateur radio.

High speed D-STAR data

On most bands the data capacity of D-STAR is relatively low as it is fundamentally a digital voice system with some data, rather than a high speed data transmission system.

However D-STAR has what is termed a Digital Data Mode, or DD mode which is supported only on 1.2 GHz as the bandwidth required is wider than that normally used for D-STAR.

This is capable of carrying data at a rate of 128 kbps. This mode requires an Ethernet connection between the PC and the D-STAR modem int he transceiver, rather than the more usual serial data cable used for the standard voice and low speed data on other bands.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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